There is now known a method of bloating a granular material to be effected in a shaft furnace (cf. "Proizvodstvo i primenenie vernikulita"--in Russian, Edited by Popov, Published by Stroiizdat, Moscow, 1964; and "Spravochnik po proizvodstvu teploizolyatsionnykh i akusticheskikh materialov"--in Russian, Edited by Kitaitsev, Published by Stroiizdat, Moscow, 1964, p. 280) residing in that raw vermiculite is admitted in countercurrent to the flow of hot gases for the thus supplied vermiculite to be roasted at a temperature of 800.degree. to 1000.degree. C. and be bloated, whereafter the bloated granules tend to fall by gravity to the lower part of the chamber of the furnace for the end product to be discharged therefrom.
In such a bloating process bloating of the vermiculite granules largely depends on the duration of their staying in the zone of high temperature, whereby during uniform heating conditions larger granules tend to underbloat, while smaller ones overroast.
Because vermiculite is supplied in countercurrent to the less heated upper portion of the flow of heated gases, it bloats less efficiently.
For the above reasons the raw vermiculite is subjected to preliminary sizing to 4-6 narrow fractions, the fractions of over 2.5 mm being further sized to 0.5 mm. Each of the thus sized fractions is then subjected separately to roasting, temperature conditions being selected accordingly.
The thus obtained bloated vermiculite is too brittle due to the presence therein of overroasted small granules, the bloating rate thereof being within a range of from 5 to 6.
A shaft furnace wherein this method is carried out is provided with a lined vertical chamber of 12 m in height and up to 1 m in diameter, the lower portion of this chamber accommodating a means for burning a fuel and discharging the end product; the upper portion thereof accommodating a means for discharging combustion products and supplying the raw material. The chambers are provided with inclined trays for the raw vermiculite to stay longer in the high temperature zone.
With the number of fractions being roasted ranging from 4 to 6, there are provided a corresponding number of containers of sufficient capacity for storing such fractions and separate admission thereof to the shaft furnace. A like number of tanks and conveyors are required for receiving the end product.
The above furnace is inherently disadvantageous in low end product output and much materials consumed for its manufacture.
There is further known a method of obtaining a bloated material of the vermiculite type (cf. USSR Inventor's Certificate No. 187,605) residing in that a flow of rising gases heated to a temperature of 800.degree.-1000.degree. C. and a flow of raw granular material are formed, whereafter said material is subjected to roasting by virtue of heat transfer between this material and the rising flow of gases to thereby obtain a bloated material. The time of presence of the granules in the zone of high temperature is determined by the height at which the particles of the material being bloated ascend in the flow of gases, the height being determined by varying the pressure of compressed air forming this flow. The thus obtained material consisting of completely bloated, partially bloated, and non-bloated granules is pneumatically conveyed together with exit gases away from the zone of roasting.
A unit for carrying out the above method comprises:
a vertical furnace for bloating the granular material;
a housing of said furnace;
a chamber disposed axially in said housing of said furnace lengthwise thereof;
a reflector fashioned as a cone and located to overlie the chamber coaxially relative to its vertical centerline;
a burner of streamlined shape for combusting a fuel arranged inside a nozzle for feeding the granular raw material and mounted at a lower end of the chamber coaxially with the vertical centerline thereof; and
collector pockets disposed concentrically adjacent to an upper end of the furnace chamber.
When the raw vermiculite is pneumatically conveyed upwards of the furnace chamber, the velocity of the gas flow and the velocity of the raw material supplied are practically equal, whereby it is impossible to introduce various size fractions of the raw material to various required heights for efficient bloating. At a velocity sufficient for the larger granules to reach the necessary height relative to the conical reflector, medium size granules are entrained by the bloated granules short of getting bloated.
Larger granules fail to stay suspended in the furnace chamber even at a considerable length thereof (up to 6 m) whereby they fall down to the lower portion of the chamber short of being bloated. For more efficient bloating of the larger granules, it is possible to increase temperature in the furnace chamber to over 1000.degree. C., although this leads to overroasting of the small size granules.
The resulting mixture comprises bloated and non-bloated vermiculite granules of relatively low quality with a low output in terms of overall volume.
During pneumatically conveying the granules of the raw material by the flow of hot gases the conical reflector serves to exclusively to direct this flow and the mixture of completely and partially bloated vermiculite granules into the collector pockets, which affects the quality of the end product.
Also, a large volumetric amount of gases is required for conveying vermiculite along the furnace chamber; a subsequent cleaning of these gases from pulverulent vermiculite requiring the use of complicated devices.
Further, the aforedescribed unit features a relatively low efficiency, while a considerable amount of materials needs to be consumed for its manufacture.